Method to fabricate interconnect structures

ABSTRACT

A method includes forming a barrier layer on a substrate surface including at least one contact opening; forming an interconnect in the contact opening; and reducing the electrical conductivity of the barrier layer. A method including forming a barrier layer on a substrate surface including a dielectric layer and a contact opening, depositing a conductive material in the contact opening, removing the conductive material sufficient to expose the barrier layer on the substrate surface, and reducing the electrical conductivity of the barrier layer. An apparatus including a circuit substrate including at least one active layer including at least one contact point, a dielectric layer on the at least one active layer, a barrier layer on a surface of the dielectric layer, a portion of the barrier layer having been transformed from a first electrical conductivity to a second different and reduced electrical conductivity, and an interconnect coupled to the at least one contact point.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 10/748,106, filed Dec. 24, 2003.

BACKGROUND

1. Field

Circuit structures.

2. Relevant Art

Integrated circuits typically use conductive interconnections to connectindividual devices on a chip or to send or receive signals external tothe chip. A currently popular choice of interconnection material forsuch interconnections is a copper or copper alloy material.

One process used to form interconnections, particularly copper (alloy)interconnections, is a damascene process. In a damascene process, atrench is cut in a dieletric and filled with copper to form theinterconnection. A via may be in the dielectric beneath the trench witha conductive material in the via to connect the interconnection tounderlying integrated circuit devices or underlying interconnections. Inone damascene process (a “dual damascene process”), the trench and viaare each filled with copper material, by, for example, a singledeposition.

A photoresist is typically used over the dielectric to pattern a via ora trench or both in the dielectric for the interconnection. Afterpatterning, the photoresist is removed. The photoresist is typicallyremoved by oxygen plasma (oxygen ashing). The oxygen used in the oxygenashing can react with an underlying copper interconnection and oxidizethe interconnection. Accordingly, damascene processes typically employ abarrier layer of silicon nitride (Si₃N₄) directly over the copperinterconnection to protect the copper from oxidation during oxygenashing in the formation of a subsequent level interconnection. Ininterlayer interconnection levels (e.g., beyond a first level over adevice substrate), the barrier layer also protects against misguided orunlanded vias extending to an underlying dielectric layer or level (e.g.the barrier layer serves as an etch stop)

In general, the Si₃N₄ barrier layer is very thin, for example, roughly10 percent of the thickness of an interlayer dielectric (ILD) layer. Athin barrier layer is preferred primarily because Si₃N₄ has a relativelyhigh dielectric constant (k) on the order of 6 to 7. The dielectricconstant of a dielectric material, such as an interlayer dielectric,generally describes the parasitic capacitance of the material. As theparasitic capacitance is reduced, the cross talk (e.g., thecharacterization of the electric field between adjacentinterconnections) is reduced as is the resistance-capacitance (RC) timedelay and power consumption. Thus, the effective dielectric constant(k_(eff)) of an ILD layer is defined by the thin barrier layer andanother dielectric material having a lower dielectric constant so thatthe effect of the dielectric material typically used for the barrierlayer (e.g., Si₃N₄) is minimized.

In prior art integrated circuit structures, a popular dielectricmaterial for use in combination with a barrier layer to form ILD layerswas silicon dioxide (SiO₂). Currently, efforts have focused atminimizing the effective dielectric constant of an ILD layer somaterials having a dielectric constant lower than SiO₂ have garneredsignificant consideration. Many of these materials, such as carbon dopedoxide (CDO), are porous. The dielectric constant of a dielectricmaterial can be substantially effected by water or liquid absorbed inthe pores of the dielectric material.

A typical part of a damascene process to form an interconnection in anILD layer is a planarization after a deposition of the interconnectmaterial. A typical planarization is a chemical mechanical polish (CMP).A CMP is a wet process that can introduce water or other liquid into aporous dielectric material. In addition, it can add mechanical stress toa dielectric layer. Stress can damage a dielectric layer and effectcircuit performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional side view of a portion of acircuit structure including an interconnect trench and interconnectmaterial formed in the interconnect trench and on a surface of thesubstrate.

FIG. 2 shows the structure of FIG. 1 following the removal of theinterconnect material from the surface of the substrate.

FIG. 3 shows the structure of FIG. 2 following the introduction ofadditional interconnect material in the trench and capping of thetrench.

FIG. 4 shows the structure of FIG. 3 and a reduction of the electricalconductivity of the barrier material on the surface of the substrate.

FIG. 5 shows the structure of FIG. 4 following the introduction of asubsequent dielectric layer and a trench via formed into the dielectriclayer to the underlying interconnect.

FIG. 6 shows the structure of FIG. 5 following the formation of aninterconnect in the subsequent layer of dielectric.

The features of the described embodiments are specifically set forth inthe appended claims. Referring to the following description andaccompanying drawings, in which similar parts are identified by likereference numerals, best understand the embodiments.

DETAILED DESCRIPTION

FIG. 1 illustrates a cross-sectional, schematic side view of a portionof a circuit substrate structure. Structure 100 include substrate 110of, for example, a semiconductor material such as silicon or asemiconductor layer on an insulator such as glass. Substrate 110includes contact point 120 on a surface thereof. In one embodiment,contact point 120 is a device on a substrate (e.g., gate or junction ofa transistor, etc.) or a portion of an underlying interconnect line(e.g., a metal trench).

Overlying a superior surface of substrate 110 (as viewed) is dielectricmaterial 130. In one embodiment, dielectric material 130 is a dielectricmaterial having a dielectric constant less than the dielectric constantof silicon dioxide (k_(SiO2)=3.9), a “low k dielectric.” A suitablematerial is, for example, carbon doped oxide (CDO). Dielectric layer 130is deposited to a desired thickness, such as a thickness suitable toelectrically insulate substrate 110 (e.g., devices on or above substrate110) and to permit an interconnection to be formed therein.

FIG. 1 shows trench 140 formed in dielectric layer 130. Trench 140extends, as viewed, into and/or out of the page and indicates a locationfor an interconnect line. A via would typically be present, though notshown in this cross-section, to contact point 120. Trench 140 and anyvias formed to contact points on substrate 110 (e.g., contact point 120,etc.) may be formed, for example, through photo-lithographic patterningtechniques.

FIG. 1 shows barrier layer 150 formed on a surface of dielectric layer130 (a superior surface as viewed). Barrier layer 150 is conformallydeposited on the surface and conforms to the contours of trench 140. Inone embodiment, barrier layer 150 is of a material selected to inhibitthe diffusion of an interconnection material (e.g., a copper material)formed in trench 140 from diffusing into dielectirc layer 130. Asuitable material for barrier layer 150 includes conductive materialssuch as tantalum (Ta), tantalum nitride (TaN), titanium (Ti), molybdenum(Mo), and niobium (Nb). The thickness of barrier layer 150 may besuitable to inhibit diffusion of, for example, a copper interconnect. Athickness on the order of up to about 20 angstroms is suitable in oneembodiment.

In one embodiment, the structure of FIG. 1 also includes seed layer 160.Seed layer 160 is suitable, for example, where an electroplating processwill be used to form the interconnection. For an interconnectionmaterial of a copper material, seed layer 160 is a material that willfacilitate a copper plating process (e.g., will provide a material towhich copper will plate to). Representatively, seed layer 160 is acopper material deposited by chemical or physical deposition techniques.A thickness on the order of less than about 3,000 Å is suitable in oneembodiment. FIG. 1 shows seed layer 160 conformally deposited on barrierlayer 150 along the sidewalls and bottom of trench 140 and on barrierlayer 150 outside trench 140.

FIG. 1 shows structure 100 after filling trench 140 with interconnectmaterial 170 of, for example, a copper material (e.g., copper or acopper alloy). Suitable copper alloys include, but are not limited to,copper-tin (CuSn), copper-indium (CuIn), copper-cadmium (CuCd),copper-bismuth (CuBi), copper-rutherium (CuRu), copper-rhodium (CuRh),copper-rhenium (CuRe), and copper-tungsten (CuW). A typical introductiontechnique for a copper interconnection material as noted above is anelectroplating process. By way of example, a typical electroplatingprocess involves introducing a substrate (e.g., a wafer) into an aqueoussolution containing metal ions, such as a copper sulfate-based solution,and reducing the ions (reducing the oxidation number) to a metallicstate by applying current between a substrate with seed material (seedlayer 160) and an anode of an electroplating cell in the presence of thesolution. Referring to FIG. 1, interconnect material 170 is deposited onseed material 160 to fill trench 140 (and any vias formed between trench140 and substrate 110) and on a surface of structure 100 outside trench140 (referred to hereinafter as a field region).

FIG. 2 shows the structure of FIG. 1 following the removal ofinterconnect material 170 from the field region (i.e., from areasoutside trench 140). In the example where interconnect material 170 iscopper or a copper alloy, the material in the field region may beremoved by an electropolishing process. In one sense, electropolishingmay be thought of as the reverse of plating. In other words, instead ofdepositing copper or a copper alloy as in a plating process, anelectropolishing process electro-chemically dissolves the copper orcopper alloy. In the example of copper or a copper alloy, anelectropolishing process may be performed by polarizing structure 100annodically in a phosphoric acid solution (e.g., a concentrated (e.g.,85 percent) phosphoric acid solution). A concentrated phosphoric acidsolution is generally highly viscous which aids in the electropolishingprocess. Other suitable electropolishing solutions include a combinationof concentrated phosphoric acid and concentrated sulfuric acid, chromicacid, or acetic acid. The electropolishing solution may also includeadditional additives. Suitable additives include, but are not limitedto, viscosity modifiers such as glycerine; wetting agents such aspolyethylene glycol or polypropylene glycol; and film-forming agentssuch as phosphates or poly-phosphates. In one embodiment, as part of anelectropolishing process, structure 100 may be polarized with a voltageof 0.8 to 1.8 volts versus a saturated calomel electrode.

Referring to FIG. 2, the electropolishing process removes interconnectmaterial 170 (e.g., copper material) from the field region and, in oneembodiment, also partially in trench 140 to recess interconnect material170 in trench 140. One reason to recess trench material 140 at thisstage is to ensure that any interconnect material has been removed fromthe field region. FIG. 2 shows interconnect material 170 recessed intrench 140. FIG. 2 also shows that interconnect material 170 has beenremoved from the field region as has seed material 160 through theelectropolishing process (e.g., particularly where a material forinterconnect material 170 and seed layer 160 are similar (e.g.,copper)). Thus, FIG. 2 shows barrier layer 150 exposed in the fieldregion.

FIG. 3 shows the structure of FIG. 2 following the introduction ofadditional interconnect material in trench 140. In one embodiment, whereinterconnect material 170 is copper or a copper alloy, supplementalinterconnect material 180 is a similar material. One way to depositsupplemental interconnect material 180 is through a chemically-inducedoxidation-reduction reaction also referred to herein as electrolessplating. Unlike an electroplating process, an electroless platingprocess is not accomplished by an externally-supplied current, butinstead relies on the constituents of the plating process (e.g.,constituents of a plating bath) to initiate and carry out the platingprocess. One technique involves placing structure 100 in a bathcontaining one or more metal ions to be plated or introduced ontointerconnect material 170. In the case of introducing a copper or copperalloy as subsequent interconnect material 180, the copper is in an ionicstate having a positive oxidation number. Representatively, copper ionsmay be present in a bath as copper sulfate in a concentration range offive to 10 grams per liter (g/l). As such, the copper ions are in asense subsequent interconnect material precursors. Additional precursorsto form an alloy may include, but are not limited to, tin, indium,cadmium, etc.

Without wishing to be bound by theory, it is believed that the exposedconductive surface, in this case a conductive surface of interconnectmaterial 170, on structure 100, when exposed to the components of anelectroless plating solution or bath, undergo an oxidation-reduction(REDOX) reaction. The oxidation number of the metal ions of theintroduced subsequent interconnect material precursors are reduced whilethe oxidation number of the reducing agent(s) are increased. Suitablereducing agents therefore are included in an electroless plating bath.In one embodiment, the reducing agents are alkaline metal-free reducingagent such as formaldehyde and a pH adjuster such as tetramethylammonium hydroxide (TMAH). In one embodiment, a complexing agent such asethylene diamine tetra-acetic acid (EDTA) is also present. Arepresentative temperature of a suitable bath to electrolessly depositcopper is on the order of greater than 50° C. and a suitable pH is inthe range of 10-13. As shown in FIG. 3, according to this process, theelectroless deposition of subsequent interconnect material 180 may beselectively deposited in trench 140 on interconnect material 170.

Following the introduction of subsequent interconnect material 180, inone embodiment, shunt material 190 is deposited on subsequentinterconnect material 180 in trench 140. In one embodiment, shuntmaterial 190 is also deposited by an electroless plating process.Representatively, the shunt material includes cobalt or nickel, or analloy or cobalt or nickel. Suitable cobalt alloys include, but are notlimited to, cobalt-phosphorous (CoP), cobalt-boron (CoB),cobalt-phosphorous-boron COPB), cobalt-metal-phosphorous (CoMeP),cobalt-metal-boron (CoMeB), and cobalt-metal-phosphorous-boron (CoMePB).As used herein, “Me” includes, but it not limited to, nickel (Ni),copper (Cu), cadmium (Cd), zinc (Zn), gold (Au), silver (Ag), platinum(Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), chromium (Cr),molybdenum (Mo), iridium (Ir), rhenium (Re), and tungsten (W). The useof refractory metals (e.g., W, Re, Ru, Rh, Cr, Mo, Ir) tends to improvethe adhesive properties of shunt material 190 as well as the mechanicalhardness of shunt material 190. Combining Co and/Ni material with anoble metal (e.g., Au, Ag, Pt, Pd, Rh, Ru) allows the noble metals toact as a catalytic surface for the electroless plating on copperinterconnect material such as subsequent interconnect material 180.Phosphorous (P) and boron (B) tend to be added to the shunt material asa result of reducing agent oxidation. Phosphorous and boron tend toimprove the barrier and corrosion properties of the shunt material.

In terms of introducing metal ions of shunt material 180 for anelectroless plating process, metal ions (shunt material precursors) ofcobalt supplied by cobalt chloride, cobalt sulfate, etc., may beintroduced in a concentration range, in one embodiment, of about 10-70grams per liter (g/l), alone or with the addition of compoundscontaining metal ions of a desired alloy constituent (e.g., Ni, Cu,etc.). Examples of suitable additional compounds include ammoniumtungstate (for alloying with W), ammonium perrhenate (for alloying withRe), etc. A suitable concentration range for the addition compound(s)include 0.1 to 10 g/l.

To reduce the oxidation number of the metal ions, one or more reducingagents are included in an electroless plating bath. In one embodiment,the reducing agents are selected to be alkaline metal-free reducingagents such as ammonium hypophosphite, dimethylamine borate (DMAB),and/or glyoxylic acid in a concentration range of about 2 to 30 g/l. Thebath may also include one or more alkaline metal-free chelating agentssuch as citric acid, ammonium chloride, glycine, acetic acid, and/ormalonic acid in a concentration range of about 5 to 70 g/l. Stillfurther, one or more organic additives may also be included tofacilitate hydrogen evolution. Suitable organic additives includeRhodafac RE-610™, cystine, Triton X-100™, polypropyleneglycol/polyethylene glycol (in a molecular range of approximately 200 to10,000) and a concentration range of about 0.01 to 5 grams per liter(g/l), an alkaline, metal-free pH adjuster such as ammonium hydroxide,tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide,tatrapropyl ammonium hydroxide, and/or tetrabutyl ammonium hydroxide,may further be included in the bath to achieve a suitable pH range, suchas a pH range of 3 to 14. A representative process temperature for anelectroless plating bath such as described is on the order of 30 to 90°C. The electroless plating process introduces (e.g., plates) shuntmaterial 190 to expose conductive surfaces amenable to the platingreaction. In one embodiment, the conductive surface is limited tosubsequent interconnect material 180. Prior to the plating operation, anexposed surface of subsequent conducting material 180 may be treated toimprove the uniformity of the electroless plating of shunt material 190.Subsequent interconnect material 180 may be surface treated with anagent such as a 1 to 20 percent by volume hydrofluoric acid (HF),sulfuric acid (H₂SO₄), sulfonic acids such as methane sulfonic acid(MSA), ethane sulfonic acid (ESA), propane sulfonic acid (PSA) and/orbenzene sulfonic acid (BSA) for cleaning of the interconnect material.

FIG. 3 shows an interconnect structure including interconnect material170, subsequent interconnect material 180, and shunt material 190 as acap or overlying structure. As a non-limiting example, shunt material190 has a thickness on the order of 5 to 300 nanometers (nm).

In the structure described with reference to FIG. 3, barrier layer 150remained in the field region after the electropolishing of interconnectmaterial 170. One reason the barrier layer remained in the field regionis that the electropolishing process was selective for removal of theinterconnect material (e.g., selective for copper). In one embodiment,barrier layer 150 is an electrically conductive material, such astantalum or tantalum nitride. Thus, where barrier layer 150 remains inthe field region, it may be desirable to reduce or minimize theelectrical conductivity of a material for barrier layer 150.

FIG. 4 shows the structure of FIG. 3 following the reduction orminimization of electrical conductivity of a material for barrier layer150 in the field region. In one embodiment, the reduction orminimization of electrical conductivity of a material for barrier layer150 may be accomplished through an oxidation of the material.Representatively, structure 100 may be placed in an oxygen-containingenvironment (e.g., an oxygen plasma environment) under temperatureconditions greater than, for example, 300° C. FIG. 4 shows structure 100including barrier layer 250 of oxidized material for the barrier layerin the field region. A subsequent CMP can be introduced to planarizeelectroless plated shunt layer 190 and avoid topography build up in thenext level interconnect structure

FIG. 5 shows the structure of FIG. 4 following the introduction (e.g.,deposition) of a dielectric layer on barrier layer 250 and the compositeinterconnect (on shunt layer 190). Representatively, dielectric layer230 is a low k dielectric material (e.g., CDO) formed to a thicknesssuitable to electrically insulate the composite interconnect and allowthe formation of a subsequent interconnect in the dielectric layer. FIG.5 also shows via 235 and trench 240 formed in dielectric layer 230.Representatively, a mask, such as a photoresist mask, may be used todefine an area (e.g., a cross-sectional area) for a via opening and thenvia 235 may be etched with a suitable chemistry. The mask may then beremoved (such as by an oxygen plasma to remove photoresist) and a secondmask patterned to define a greater area (e.g., a greater cross-sectionalarea) for a trench opening. A subsequent etch is introduced to formtrench 240 and the second mask is removed leaving the structure shown inFIG. 5.

FIG. 5 shows via 235 as a partially unlanded via. In that sense, via 235is formed through dielectric layer 230 and contacts a portion of shuntlayer 190 indicated at point 300. A portion of via 235 also contactsbarrier layer 250. In the embodiment where barrier layer 250 has had itselectrical conductivity reduced or minimized, a partially unlanded viasuch as via 235 may not adversely effect the circuit (e.g., because theelectrical conductivity of barrier layer 250 is reduced or minimized).

FIG. 6 shows the structure of FIG. 5 following the formation of a, asillustrated, second level interconnect structure of a compositestructure including electroplated interconnect material 270 subsequentlyintroduced (electrolessly plated) interconnect material 280 and shuntlayer 290. FIG. 6 also shows a subsequent barrier layer 350 with itselectrical conductivity minimized or reduced. It is appreciated that theprocess described with respect to FIGS. 1-6 may be repeated for multipleinterconnect levels.

In the preceding detailed description, specific embodiments weredescribed. It will, however, be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope as set forth in the claims. The specification and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

1. An apparatus comprising: a circuit substrate comprising at least oneactive layer including at least one contact point; a dielectric layer onthe at least one active layer with an opening to the contact point; abarrier layer on a surface of the dielectric layer and in the opening, aportion of the barrier layer on the surface having a first electricalconductivity that is less than a second electrical conductivity of aportion of the barrier layer in the opening; and an interconnectdisposed in the opening through the dielectric layer and coupled to theat least one contact point.
 2. The apparatus of claim 1, wherein thedielectric layer comprises a material having a dielectric constant thatis less than a dielectric constant of silicon dioxide.
 3. The apparatusof claim 1, wherein a material for the first portion of the barrierlayer is an oxidized form of a material for the second portion of thebarrier layer.
 4. The apparatus of claim 1 further comprising a caplayer on the interconnect, wherein the interconnect is defined betweenthe cap layer and the contact point.
 5. The apparatus of claim 4,wherein the cap layer extends an area limited by a surface area of theinterconnect.
 6. The apparatus of claim 4, wherein the cap layercomprises a refractory metal.
 7. The apparatus of claim 4, wherein thecap layer comprises an alloy of one of cobalt and nickel.
 8. Theapparatus of claim 7, wherein the alloy comprises a noble metal.
 9. Theapparatus of claim 4, wherein the cap layer comprises one of boron andphosphorous.
 10. The apparatus of claim 1, wherein the dielectric layercomprises a first dielectric layer, the apparatus further comprising atleast a second dielectric layer formed on the first dielectric layer.